when do I use 3/2R??


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Hailey Boehm 2H
Posts: 71
Joined: Fri Sep 28, 2018 12:24 am

when do I use 3/2R??

Postby Hailey Boehm 2H » Tue Feb 12, 2019 9:04 pm

I understand that 3/2*R is used in entropy calculations for ideal monatomic gases, but I am unsure exactly as to when I should use it versus just R. When exactly do I use it?

Becky Belisle 1A
Posts: 81
Joined: Fri Sep 28, 2018 12:18 am

Re: when do I use 3/2R??

Postby Becky Belisle 1A » Tue Feb 12, 2019 9:08 pm

3/2R is the C value at a constant volume. It can be used in the equation ΔS=nCln(T1/T2). In this equation, C can be substituted with Cp (constant pressure) or Cv (constant volume) depending on the problem.

Erin Kim 2G
Posts: 75
Joined: Fri Sep 28, 2018 12:26 am

Re: when do I use 3/2R??

Postby Erin Kim 2G » Tue Feb 12, 2019 9:52 pm

Change in Entropy= nCln(T2/T1). When volume is constant the value C is Cv and is equal to 3/2R. This is also the case for monatomic gases.

Irene Zhou 1E
Posts: 73
Joined: Fri Sep 28, 2018 12:18 am

Re: when do I use 3/2R??

Postby Irene Zhou 1E » Tue Feb 12, 2019 9:56 pm

You would use (3/2)R to calculate the heat capacity, Cvm, of a monoatomic gas when it is at constant volume. You would use (5/2)R to calculate the heat capacity, Cpm, of a gas at constant pressure.

Michael Novelo 4G
Posts: 64
Joined: Fri Sep 28, 2018 12:28 am

Re: when do I use 3/2R??

Postby Michael Novelo 4G » Tue Feb 12, 2019 9:59 pm

It's commonly used in ΔS=nCln(T1/T2) when the questions states its an ideal gas or monatomic gas, if the question ask to calculate entropy with constant volume (Cv then use 3/2R) if it is constant pressure (Cp then use 5/2R) and R is based on the units of measurement.


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